Postage meters are mass produced devices for printing a defined unit value for governmental or private carrier delivery of parcel and envelopes. The term "postage meter" also includes other like devices which provide unit value printing such as tax stamp meters. Postage meters include internal accounting devices which account for postage value representation which is stored within the meter and is printed by the meter. As a result, postage meters must possess an extremely high reliability to avoid the loss of user or governmental funds stored within the meters.
Electronic postage meters have been developed with electronic accounting circuitry. Postage meter systems of this type are disclosed in U.S. Pat. No. 3,978,457 for "MICROCOMPUTERIZED ELECTRONIC POSTAGE METER SYSTEM" and U.S. Pat. No. 4,301,507 for "ELECTRONIC POSTAGE METER HAVING PLURAL COMPUTING SYSTEM". The electronic postage meters of this type include non-volatile memory capability for storing postage accounting information within the meter. The memory function in the meter electronic accounting circuits have replaced the function served in mechanical postage meters by mechanical accounting registers.
Electronic postage meter systems have also been proposed that employ encrypting techniques. Systems of this type are disclosed in U.S. patent application Ser. No. 724,372 filed Apr. 17, 1985, for George B. Edelmann and Arno Muller and entitled "SYSTEM FOR DETECTING UNACCOUNTED FOR PRINTING IN A VALUE PRINTING SYSTEM", now U.S. Pat. No. 4,757,537 an additional continuation in part application for Ser. No. 832,904 filed Feb. 25, 1986 for George B. Edelmann, Arno Muller, Alfred Schmidt and Kevin Hunter and now U.S. Pat. No. 4,775,246 also describes systems employing encryption techniques in printing postage. In these systems, the metering device at the user location includes a stored representation within the meter of the postage value available for printing. Printing is accomplished by a type of printing device which prints variable data, in this case both the postage value and encrypted information so that the validity of the imprint can be verified at a later date.